The next generation mobile wireless communication system (5G or NR), will support a diverse set of use cases and a diverse set of deployment scenarios. The later includes deployment at both low frequencies (100 s of MHz), similar to LTE today, and very high frequencies (mm waves in the tens of GHz). At high frequencies, propagation characteristics make achieving good coverage challenging. One solution to the coverage issue is to employ high-gain beamforming, typically in an analog manner, in order to achieve satisfactory link budget. Beamforming will also be used at lower frequencies (typically digital beamforming), and is expected to be similar in nature to the already standardized 3GPP LTE system (4G).
Moreover, it is expected that large parts of future NR networks will be deployed for TDD. One benefit with TDD (compared to FDD) is that TDD enables reciprocity based beamforming, which can be applied both at the TRP (i.e. for DL) and the UE (i.e. for UL). For reciprocity based DL transmission it is expected that the UE will transmit Sounding Reference Signals (SRSs) which the TRP will use to estimate the channel between the TRP and UE. The channel estimate will then be used at the TRP to find optimal precoding weights for the coming DL transmission, for example by using eigenbeamforming. In similar way, it is expected that CSI-RS will be used as sounding signal for reciprocity based UL transmissions. It has been agreed in NR that a TRP can indicate a quasi co-location (QCL) assumption to an earlier transmitted DL reference signal (e.g. CSI-RS) that a UE may use when determining UL precoding.
Codebook-Based Precoding
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The NR standard is currently being specified. A core component in NR is the support of MIMO antenna deployments and MIMO related techniques. It is expected that NR will support uplink MIMO with at least 4 layer spatial multiplexing using at least 4 antenna ports with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 4 for where CP-OFDM is used on the uplink.
As seen, the information carrying symbol vector s is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE and NR uses OFDM in the downlink and hence the received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en where en is a noise/interference vector obtained as realizations of a random process. The precoder implemented by precoder matrix, W, can be a wideband precoder, which is constant over frequency, or frequency selective.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
One example method for a UE to select a precoder matrix W can be to select the Wk that maximizes the Frobenius norm of the hypothesized equivalent channel:
      max    k    ⁢                                                H            ^                    n                ⁢                  W          k                            F    2  where
Ĥn is a channel estimate, possibly derived from CSI-RS as described further below,
Wk is a hypothesized precoder matrix with index k, and
ĤnWk is the hypothesized equivalent channel.
In closed-loop precoding for the NR uplink, the TRP transmits, based on channel measurements in the reverse link (uplink), TPMI to the UE that the UE should use on its uplink antennas. The gNodeB configures the UE to transmit SRS according to the number of UE antennas it would like the UE to use for uplink transmission to enable the channel measurements. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be signaled. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders and/or several TPMIs, one per subband.
Other information than TPMI is generally used to determine the UL MIMO transmission state, such as SRS resource indicators (SRIs) as well as transmission rank indicator (TRIs). These parameters, as well as the modulation and coding state (MCS), and the uplink resources where PUSCH is to be transmitted, are also determined by channel measurements derived from SRS transmissions from the UE. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder W. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
Non-Codebook Based UL Transmission
In addition to codebook-based UL transmission, it has been agreed that NR will support a non-codebook based transmission modes, which is applicable when TX/RX reciprocity holds at the UE. In the codebook-based mode, as stated earlier, the UE typically transmits a non-precoded SRS to sound the uplink channel and the gNB determines a preferred precoder from the codebook based on the SRS channel estimates and instructs the UE to apply said precoder on the PUSCH transmission by means of a TPMI comprised in the UL grant.
For non-codebook based UL transmission however, the UE itself determines one or more precoder candidates and uses said precoder candidates to precode one or more SRS in one or more SRS resources. The gNB correspondingly determines one or more preferred SRS resource and instructs the UE to use the precoder(s) applied for precoding the one or more preferred SRS resources also for the PUSCH transmission. This instruction may be signaled in the form of one or more SRI(s) comprised in the DCI carrying the UL grant, but may alternatively or additionally include TRI signaling.
For the UE to determine the UL precoder candidates, it needs to measure a DL reference signal, such as a CSI-RS in order to attain a DL channel estimate. Based on this DL channel estimate, and assuming TX/RX reciprocity holds, the UE can convert the DL channel estimate into an UL channel estimate and use the UL channel estimate to determine a set of UL precoder candidates, for instance by performing a singular value decomposition (SVD) of the UL channel estimate or by other established precoder determination methods. Typically, the gNB would configure the UE, implicitly or explicitly, with which CSI-RS resource it can use to aid precoder candidate determination. In some proposals for NR, this is done by indicating that a certain CSI-RS resource is reciprocally spatially quasi co-located with the SRS resource(s) the UE is scheduled to use for UL sounding, for instance as a part of RRC configuration.
SRS Transmission Setting
How the SRS transmission should be done, for example which SRS resource to use, the number of ports per SRS resource, etc, needs to be signaled to the UE from the TRP. One way to solve this (in a low overhead way) is to pre-define a set of “SRS transmission settings” using higher layer signaling (e.g. RRC) and then indicate in DCI which “SRS transmission setting” that the UE should apply. An “SRS transmission setting” can for example contain information regarding which SRS resources and SRS ports that the UE should use in the coming SRS transmission.
Exactly how SRS transmissions are configured and triggered for NR is still under discussion, a text proposal to 3GPP Technical Specification 38.331 defining the SRS related parameters are given in FIG. 24.
As shown in FIG. 24, the SRS-Config IE is used to configure sounding reference signal transmissions. The configuration defines a list of SRS-Resources and a list of SRS-ResourceSets. Each resource set defines a set of SRS-Resources. The network triggers the transmission of the set of SRS-Resources using a configured aperiodicSRS-ResourceTrigger (that is carried in physical layer downlink control information, ‘L1 DCI’).
Thus, the RRC configuration of “SRS transmission settings” are done with the IE SRS-Config, which contains a list of SRS-Resources (the list constitutes a “pool” of resources) wherein each SRS resource contains information of the physical mapping of the reference signal on the time-frequency grid, time-domain information, sequence IDs, etc. The SRS-Config also contains a list of SRS resource sets, which contains a list of SRS resources and an associated DCI trigger state. Thus, when a certain DCI state is triggered, it indicates that the SRS resources in the associated set shall be transmitted by the UE.
UL Beam Management
Concepts for UL beam management (i.e. beam management based on UL reference signals) are currently being developed for NR to control the beam (or more correctly the effective antenna pattern) for a respective UE panel. It is expected that UL beam management is performed by letting the UE transmit different SRS resources in different UE panel beams, which the TRP performs RSRP measurements on and signals back the SRI(s) corresponding to the SRS resource(s) with highest RSRP value(s). If a multi-panel UE is scheduled for SRS transmission of multiple beams from each of the multiple panels, the TRP and UE need to have a mutual agreement of which combinations of SRS resources can be transmitted simultaneously from the different panels. Otherwise the TRP could select SRS resources that could not be transmitted simultaneously, such as when the SRS resources correspond to different switched analog beams in the same panel. The following note to the agreement from RAN1#90 for signaling multiple SRIs (below) addresses this issue but does not conclude on how it should be done. Note: The gNB should only signal SRI(s) such that the UL precoding transmission inferred from the signaled SRI(s) can be simultaneously conducted by the UE.